1. Field of the Invention
The present invention relates to a receiving apparatus and a receiving method, a program, and a receiving system, and more particularly to a receiving apparatus and a receiving method each of which can precisely estimate a transmission path even in the case of a frame in which one frame length is short, a program, and a receiving system.
2. Description of the Related Art
In recent years, a modulation system called an orthogonal frequency division multiplexing (OFDM) system has been used as a system for transmitting a digital signal. The OFDM system is a system in which a large number of orthogonal subcarriers are prepared within a transmission band, and data is allocated to amplitudes and phases of respective subcarriers, thereby carrying out digital modulation in accordance with either Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM). An OFDM time domain signal is transmitted in units of a symbol called an OFDM symbol.
The OFDM system is applied to a terrestrial digital broadcasting which strongly receives an influence of a multipath interference in many cases. For example, the standards such as a Digital Video Broadcasting-Terrestrial (DVB-T) and an Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) are known as the standards for the terrestrial digital broadcasting adopting such an OFDM system.
In a transmission system adopting such an OFDM system, a known signal called a Scattered Pilot (SP) is inserted into the data.
FIG. 1 shows a pattern of arrangement of pilots within the OFDM symbol. In an example of FIG. 1, one circle mark represents one OFDM symbol. Also, an axis of abscissa represents a carrier number (carr num) of the OFDM signal, and an axis of ordinate represents a symbol number (sym num) of the OFDM signal. In addition, an open circle mark represents data (carrier) becoming an object of the transmission, and a back circle mark represents a pilot (either an Edge pilot or an SP). That is to say, the pilots located in the carriers each having a carrier number 0 are the Edge pilots.
As shown in FIG. 1, in the OFDM symbol, the SP is inserted once every 12 carriers in a carrier direction, and is inserted once every 4 symbols in a symbol direction. Also, one frame length is decided as 68 symbols in the DVB-T, and is decided as 204 symbols in the ISDM-T.
In the receiving apparatus in such a transmission system, a method of carrying out an interpolation in a time direction by using the SPs arranged in the same carrier, thereby carrying out estimation of a transmission path is generally known.
FIG. 2 shows an example after completion of the time interpolation made by using the SPs shown in FIG. 1. In the example of FIG. 2, hatched portions (for example, carrier numbers 0, 3, 6, 9, . . . ) represent the respective carriers interpolated by using the SPs.
Now, at the present time (as of May, 2009), Digital Video Broadcasting (DVB)-T.2 is being enacted as the standard of the next generation terrestrial digital broadcasting by the European Telecommunication Standard Institute (ETSI). This is described in a Non-Patent Document of DVB BlueBook A122 Rev. 1, Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2) Sep. 1, 2008, home page of DVB, [retrieved on Nov. 10, 2009], the Internet <URL: http://www.dvb.org/technology/standards/].
In the DVB-T2, unlike the DVB-T or the ISDB-T, the arrangement of the SPs is not unique. As shown in FIGS. 3 to 5, the arrangement of the SPs inserted into the data is decided from PP1 to PP8 as a Pilot Pattern (PP).
FIG. 3 and FIGS. 4A to 4H show examples of the arrangement of the SPs in PP1 to PP8. Character Dx in FIG. 3 represents a time interpolation interval, character Dy represents a symbol interval of the SPs in the same carrier, and Dx×Dy represents an interval of the SPs appearing in the same symbol. Each of the examples of FIGS. 4A to 4H is similar to that of FIG. 1. That is, an open circle mark represents the data becoming an object of the transmission, and a black circle mark represents the pilot (either the Edge pilot or the SP).
In the case of PP1 shown in FIG. 4A, Dx=3, Dy=4, and Dx×Dy=12. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 4 . . . in the carrier numbers 12, 24, . . . , and in the symbols having the symbol numbers 1, 5, 9, . . . in the carrier numbers 3, 15, . . . . In addition, the SPs are respectively arranged in the symbols having the symbol numbers 2, 6, . . . in the carrier numbers 6, 18, . . . , and in the symbols having the symbol numbers 3, 7, . . . in the carrier numbers 9, 21, . . . .
In the case of PP2 shown in FIG. 4B, Dx=6, Dy=2, and Dx×Dy=12. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 2, 4, 6, . . . in the carrier numbers 12, 24, . . . , and in the symbols having the symbol numbers 1, 3, 5, 7, . . . in the carrier numbers 6, 18, . . . .
In the case of PP3 shown in FIG. 4C, Dx=6, Dy=4, and Dx×Dy=24. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 4, . . . in the carrier numbers 24, 48, . . . and in the symbols having the symbol numbers 1, 5, . . . in the carrier numbers 6, 30, . . . . In addition, the SPs are respectively arranged in the symbols having the symbol numbers 2, 6, . . . in the carrier numbers 12, 36, . . . , and in the symbols having the symbol numbers 3, 7, . . . in the carrier numbers 18, 42, . . . .
In the case of PP4 shown in FIG. 4D, Dx=12, Dy=2, and Dx×Dy=24. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 2, 4, 6, . . . in the carrier numbers 24, 48, . . . , and in the symbols having the symbol numbers 1, 3, 5, 7, . . . in the carrier numbers 12, 36, . . . .
In the case of PP5 shown in FIG. 4E, Dx=12, Dy=4, and Dx×Dy=48. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 4 . . . in the carrier numbers 48, 96, . . . , and in the symbols having the symbol numbers 1, 5, . . . in the carrier numbers 12, 60, . . . . In addition, the SPs are respectively arranged in the symbols having the symbol numbers 2, 6, . . . in the carrier numbers 24, 72, . . . , and in the symbols having the symbol numbers 3, 7, . . . in the carrier numbers 36, 84, . . . .
In the case of PP6 shown in FIG. 4F, Dx=24, Dy=2, and Dx×Dy=48. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 2, 4, 6, . . . in the carrier numbers 48, 96, . . . , and in the symbols having the symbol numbers 1, 3, 5, 7, . . . in the carrier numbers 24, 72, . . . .
In the case of PP7 shown in FIG. 4G, Dx=24, Dy=4, and Dx×Dy=96. When the carrier number 0 is excluded because the Edge pilots are arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 4, . . . in the carrier numbers 96, 192, . . . and in the symbols having the symbol numbers 1, 5, . . . and in the carrier numbers 24, 120, . . . . In addition, the SPs are respectively arranged in the symbols having the symbol numbers 2, 6, . . . in the carrier numbers 48, 144, . . . , and in the symbols having the symbol numbers 3, 7, . . . in the carrier numbers 72, 168, . . . .
In the case of PP8 shown in FIG. 4H, Dx=6, Dy=16, and Dx×Dy=96. When the carrier number 0 is excluded because the Edge pilots are respectively arranged in the carrier number 0, the SPs are respectively arranged in the symbols having the symbol numbers 0, 16, . . . in the carrier numbers 96, 192, . . . , and in the symbols having the symbol numbers 1, 17, . . . in the carrier numbers 6, 102, . . . . In addition, the SPs are respectively arranged in the symbols having the symbol numbers 2, 18, . . . in the carrier numbers 12, 108, . . . , in the symbols having the symbol numbers 3, 19, . . . in the carrier numbers 18, 144, . . . , and in the symbols having the symbol numbers 4, 20, . . . in the carrier numbers 24, 120, . . . .
In addition, in the case of the DVB-T2, the number of symbols in one frame is decided in its maximum/minimum number of symbols by using an FFT size and a guard interval (GI).
FIG. 5 is a diagram showing a format of a T2 frame. A P1 symbol, a P2 symbol, and a symbol referred to as Normal and a symbol referred to as Flame Closing (FC) (each of them is a data symbol) are arranged in this order in the T2 frame. It is noted that the OFDM symbol is generally composed of an effective symbol as a signal period of time for which the IFFT is carried out in a phase of modulation, and a guard interval (GI) in which a part of a waveform of the second half of the effective symbol is copied to the head of the effectively symbol as it is. In FIG. 5, a narrow portion represents the guard interval, and the P1 symbol does not have GI.
The number (Np1) of P1 symbols in one frame is set as one symbol. The number (Np2) of P2 symbols in one frame is set depending on the FFT size. Also, the number (Lf) of symbols in one frame except for P1 is (Np2+NDSYM) (the number of symbols in one frame except for P1 and P2), and its maximum value and minimum value, as shown in FIGS. 6 and 7, are decided by using both the FFT size and the GI.
It is noted that the PP, the FFT size, and the NDSYM are contained in an L1 presignaling of the P2 symbol.
FIG. 6 is a diagram showing a maximum Lf.
The maximum Lf when the FFT size is 32 K is decided in such a way that it is 68 when GI1/128, is 66 when GI1/32, is 64 when GI1/16, is 64 when GI19/256, is 60 when GI1/8, is 60 when GI19/128, and is not applicable (NA) when GI1/4. It is noted that when the FFT size is 32K, Lf is decided as an even number in terms of a remark.
The maximum Lf when the FFT size is 16K is decided in such a way that it is 138 when GI1/128, is 135 when GI1/32, is 131 when GI1/16, is 129 when GI19/256, is 123 when GI1/8, is 121 when GI19/128, and is 111 when GI1/4.
The maximum Lf when the FFT size is 8K is decided in such a way that it is 276 when GI1/128, is 270 when GI1/32, is 262 when GI1/16, is 259 when GI19/256, is 247 when GI1/8, is 242 when GI19/128, and is 223 when GI1/4.
The maximum Lf when the FFT size is 4K is decided in such a way that it is NA when GI1/128, is 540 when GI1/32, is 524 when GI1/16, is NA when GI19/256, is 495 when GI1/8, is NA when GI19/128, and is 446 when GI1/4.
The maximum Lf when the FFT size is 2K is decided in such a way that it is NA when GI1/128, is 1,081 when GI1/32, is 1049 when GI1/16, is NA when GI19/256, is 991 when GI1/8, is NA when GI19/128, and is 892 when GI1/4.
The maximum Lf when the FFT size is 1K is decided in such a way that it is NA when GI1/128, is NA when GI1/32, is 2098 when GI1/16, is NA when GI19/256, is 1,982 when GI1/8, is NA when GI19/128, and is 1,784 when GI1/4.
FIG. 7 is a diagram showing a minimum Lf.
The minimum Lf when the FFT size is 32K is (Np2+3), and Lf is decided as an even number in terms of a remark. The minimum Lf in the case of the FFT size other than 32K is decided as (Np2+7).
As described above, the arrangement of the SPs, and the number of symbols in one frame are decided in the case of the DVB-T2. As a result, Lf(Np2+NDSYM) becomes smaller than Dy, and thus the effective SPs necessary for the time interpolation become insufficient in some cases.
FIG. 8 is a diagram showing a pattern in which Lf(Np2+NDSYM) becomes smaller than Dy.
In the case where the FFT size is 32K and the PP is PP8, when Np2 is 1 and NDSYM is in the range of 3 to 13, Lf(Np2+NDSYM) becomes smaller than Dy. It is noted that in this case, Lf(Np2+NDSYM) is decided as an even number.
In the case where the FFT size is 16K and the PP is PP8, when Np2 is 1 and NDSYM is in the range of 7 to 14, Lf(Np2+NDSYM) becomes smaller than Dy.
In the case where the FFT size is 8K and the PP is PP8, when Np2 is 2 and NDSYM is in the range of 7 to 13, Lf(Np2+NDSYM) becomes smaller than Dy.
It is noted that since when the FFT size is either 2K or 4K, PP8 is not got, the conditions described above are not met. In addition, since the FFT size is 1K, PP8 is not got and thus in the first place, a minimum value of (P2+NDSYM) is 23, the conditions described above are not met.
The frame having such a short one frame length that the number of symbols in one frame except for P1 becomes smaller than the symbol interval of the SPs in the same carrier is called a short frame. Thus, in the case of the short frame, the effective SPs necessary for the time interpolation become insufficient.
FIG. 9 is a diagram showing an example of the time interpolation in the case of the short frame. It is noted that the example of FIG. 9 shows the case where the FFT size is 32K, PP8 (Dx=6, Dy=16, Np2=1), Extended mode, NDSYM=3. In the example of FIG. 9, similarly to the case of FIG. 1, an open circle mark represents the data (carrier) becoming an object of the transmission, and a black circle mark represents the pilot (either the Edge pilot or the SP). However, a dotted circle mark represents the pilot in the P2 symbol. In addition, in the example of FIG. 9, similarly to the case of FIG. 2, a hatched portion represents the carrier which is interpolated by using the SPs.
Although since in the case of this example, Dx=6, Dy=16, and Dx×Dy=96, the decision is made as shown in PP8 of FIG. 4H, only the symbols having the symbol numbers 0 to 3 exist because Np2=1 and NDSYM=3. Therefore, although the SPs are respectively arranged in the carrier numbers 0 to 18, and 96 to 114 each hatched, the SPs are not respectively arranged in the carrier numbers 24 to 90 in a section indicated by A.
That is to say, none of SPs is arranged in the section indicated by A, and thus unlike the case of the carrier numbers 0 to 18 and 96 to 114, it is difficult to carry out the time interpolation.